1. Field of the Invention
This invention relates to a hydraulic control system for controlling a hydraulic operation machine such as a hydraulic shovel.
2. Description of Related Art
FIG. 11 shows a conventional hydraulic control system, which is for controlling a hydraulic operation machine such as a hydraulic shovel.
Pump 1 is connected with a cylinder device 3 via a control valve 2.
The control valve 2 switches by a pilot pressure introduced into a pilot chamber 2a or 2b. The pilot pressure to be introduced into the pilot chambers 2a and 2b is controlled by a pilot valve 4. For example, if the pilot pressure is introduced into the pilot chamber 2a, the control valve 2 switches to an upward position a (the left side of the drawing) in proportion to the volume of the introduced pilot pressure. If the pilot pressure is introduced into the pilot chamber 2b, the control valve 2 switches to a downward position b (the right side of the drawing) in proportion to the volume of the introduced pilot pressure.
Between the control valve 2 and a bottom pressure chamber 3a of the cylinder device 3, a load supporting valve assembly 6 is arranged, and which includes a pilot check valve 7, a selector valve 9 and an overload relief valve 10.
As discussed in more detail further below, the control valve 2 is connected to a load supporting passage 5, and the pilot check valve 7 is provided on the load supporting passage 5.
The pilot check valve 7 passes flow only from the control valve 2. A check function of the pilot check valve 7 is released in communication between a pilot passage 8 and a tank.
The pilot passage 8 of the pilot check valve 7 is connected to the selector valve 9.
The selector valve 9 in a normal state shown in FIG. 11 is in a blocking position for closing the pilot passage 8. The pilot check valve 7, therefore, causes its normal check function to be active. Whereas, on introduction of the pilot pressure into a pilot chamber 9a, the selector valve 9 switches to a communicating position so as to link the pilot passage 8 with the tank, thus the check function of the pilot check valve 7 is released.
The pilot pressure in the pilot chamber 2b of the control valve 2 is introduced into the pilot chamber 9a of the selector valve 9 designed as mentioned above. That is to say, the selector valve 9 is adapted to switch to the communicating position whenever the control valve 2 switches to the downward position b (the right side of the drawing).
The overload relief valve 10 is connected to a passage between the pilot check valve 7 and the bottom pressure chamber 3a of the cylinder device 3. In a maintaining state of work load W. the overload relief valve 10 functions to prevent an extraordinary increase of a load pressure in the bottom pressure chamber 3a, thereby absorbing shock produced when an external force is applied to the work load W.
Operation of the conventional hydraulic control system will be explained below.
As shown in FIG. 11, if the control valve 2 is in a neutral position, oil discharged from the pump 1 is introduced into neither the bottom pressure chamber 3a nor a rod-side pressure chamber 3b of the cylinder device 3.
The pilot pressure is not also introduced into the pilot chamber 9a, and the selector valve 9 is in the blocking position, so that the check function of the pilot check valve 7 is inactive. Thus, the flow from the bottom pressure chamber 3a of the cylinder device 3 is blocked and the work load W is assuredly maintained.
When the work load W needs to move up, the pilot pressure is introduced from the pilot valve 4 into the pilot chamber 2a, so that the control valve 2 switches to the upward position a (the left side of the drawing).
In this time, since the selector valve 9 keeps in the blocking position, the check function of the pilot check valve 7 is active. Hence, when the discharge pressure of the pump 1 is properly increased, the oil discharged from the pump 1 opens the pilot check valve 7 and then is introduced toward the bottom pressure chamber 3a of the cylinder device 3, while the hydraulic fluid in the rod-side pressure chamber 3b is discharged to the tank, thus moving up the work load W.
On the other hand, when the work load W needs to move down, the pilot pressure is introduced from the pilot valve 4 into the pilot chamber 2b, so that the control valve 2 switches to the downward position b (the right side of the drawing).
In this time, the pilot pressure is also introduced into the pilot chamber 9a, and the selector valve 9 switches to the communication position, so that the check function of the pilot check valve 7 is inactive. Hence, the hydraulic fluid in the bottom pressure chamber 3a of the cylinder device 3 passes through the pilot check valve 7 and is discharged into the tank in accordance with the degree of opening control valve 2, thus moving down the work load W.
Regarding the hydraulic control systems of prior art, in an operation for moving an objective downward with a hydraulic shovel in a craning work, for example, if the load supporting passage 5 is fractured, the hydraulic fluid in the bottom pressure chamber 3a of the cylinder device 3 may rapidly escape from the fractured section, whereby causing the objective to rapidly fall.
In this case, by returning the position of the control valve 2 to the neutral position, the selector valve 9 switches back to the blocking position, so that the check function of the pilot check valve 7 becomes active, whereby the falling objective can be stopped.
Actually, however, the objective must crashes on the ground due to its falling speed before the control valve 2 switches to the neutral position.
In the hydraulic control system of prior art, in order to absorb shock produced when the external force is applied to the work load W, in the supporting state of the work load W, the overload relief valve 10 must be connected to the passage between the bottom pressure chamber 3a of the cylinder device 3 and the pilot check valve 7.
The load supporting valve assembly 6 is typically positioned close to the cylinder device 3. In other words, the overload relief valve 10 that is included in the load supporting valve assembly 6 is also positioned closed to the cylinder device 3, which is far away from the construction machine body.
However, the tank is typically placed around the construction machine body. Accordingly, as the overload relief valve 10 is distanced from the construction machine body, a pipe for connecting the overload relief valve 10 with the tank becomes longer.
The connecting pipe needs to be of significant capacity since the overload relief valve 10 is for the escape of overloading pressure. Therefore, such a long connecting pipe causes increases in cost and size.